1. Field of the Invention
The present invention relates to a solid-state imaging device whose dynamic range can be widened.
2. Description of the Related Art
In general, an object which is to be converted into a video signal sometimes includes a luminance range wider than the dynamic range of a solid-state image pickup element. In such a case, the image of the object cannot be correctly reproduced. Therefore, the dynamic range of the solid-state image pickup element must be widened.
Japanese Patent Application No. 63-201406 discloses a device for widening the dynamic range of a solid-state image pickup element. According to this device, a plurality of images obtained by exposure for different exposure times are read from a nondestructive read-type image pickup element. The images are stored in a plurality of memories provided outside the image pickup element and are added to widen the dynamic range.
When this method is applied to an ordinary CCD, a time corresponding to several frames is required to obtain one frame image having a wide dynamic range. For example, when an image is picked up for 5 different exposure times, a time corresponding to 5 frames is required to obtain one frame image having a wide dynamic range, and it takes time to read the image from the image pickup element.
Methods for adding a plurality of images corresponding to different exposure times in accordance with an ordinary television rate have conventionally been provided including, e.g., a drive method proposed by Published Unexamined Japanese Patent Application No. 63-232591. According to this drive method, drive pulses of timings as shown in FIG. 11 are applied to a transfer gate for reading charges accumulated in an image pickup element. The image pickup element is reset by a pulse .phi.0, and exposure is performed for a time t1. Charges accumulated during this time are transferred to a vertical shift register in response to a pulse .phi.1.
Then, exposure is performed again for a time t2. Charges accumulated during this time are transferred to the vertical shift register in response to a pulse .phi.2, and the charges transferred in response to the pulses .phi.1 and .phi.2 are added. Thereafter, charges are transferred in the same manner. When the last charges are transferred in response to a pulse .phi.5 and are added by the vertical shift register, the sum is output to outside the element during a time t0.
However, non-uniform saturation can occur in a widely used image pickup element when the light-receiving section of the element is irradiated with light of an excessive light amount. As a result, a so-called fixed pattern noise component occurs and this noise component is included in the sum. The fixed pattern noise disables accurate reproduction of an object image.
More specifically, in the image pickup element, the charges accumulated in the light-receiving section are transferred to the vertical shift register through the transfer gate, as shown in FIG. 12. Charges saturated by an excessive light amount during an exposure time are discharged to a overflow drain. Adverse effects of saturated charges are thus prevented.
However, since the height of the overflow drain varies, an actual saturation value varies.
As a result, assume that the drive pulses shown in FIG. 11 are employed. When there is a pixel that saturates by an exposure for the times t1 to t3' the sum obtained by addition of the vertical shift register includes a fixed pattern noise component.
As described above, a conventional solid-state image pickup element adds read signal charges a plurality of times in units of pixels in order to widen the dynamic range. Thus, the adder, e.g., a vertical shift register needs a charge accumulator of a very large capacity which will not be saturated even after adding transferred charges a plurality of times.
As described above, however, since the adder must be incorporated in the image pickup element, it is considerably difficult to assign a large area to the charge accumulator to increase the capacity. Therefore, a large-capacity adder which will not be saturated even after performing addition a plurality of times has been desired.